XV EGGS 4II 
are perforated at the animal pole of the egg by a small aper- 
ture or “micropyle,” which is only large enough to admit of 
the entrance of a single spermatozoon at a time (Fig. 235), 
Generally, there is only a single micropyle, but, according to 
Salensky, the Sturgeon (4. sturio) has from 3 to 9, and the 
Sterlet (4. ruthenus) from 5 to 15. 
An important distinction may be made between the ova of 
different Teleostomi as regards their location after extrusion from 
the female. From this point of view two types of ova can be 
distinguished, demersal and pelagic ova. Demersal eggs are 
characterised by their larger size and greater weight, so that they 
always sink after extrusion; and by their opacity. They may 
either have an outer egg-membrane which is viscid and adhesive, 
so that the eggs readily adhere to one another or to foreign objects, 
or the membrane is smooth and non-adhesive. The Salmonidae, 
for example, produce non-adhesive demersal eggs, which remain 
separate after being deposited on the gravelly bed of a stream. 
Most freshwater and many marine shore Fishes have adhesive 
demersal eggs, which are deposited at the bottom of the water, 
generally adhering to one another in larger or smaller clumps, 
masses, or sheets, and attached to rocks, stones, or empty shells, 
like the eggs of many shore Fishes, or to aquatic plants after the 
fashion of the eggs of the Carp, Perch, and Pike, or even to 
branching zoophytes, as is the case with the eggs of the Sea-snail 
(Liparis). In some adhesive eggs the external egg-membrane 
forms threads for their attachment. The eggs of the Gar-Fish 
(Belone), and those of the Saury Pike (Scombresor) and of the 
Flying Fishes (Zzocoetus), have viscid threads developed from 
opposite points on the surface, which are either attached to 
foreien objects or they become entangled with those of other 
egos of the same species. The oval eggs of some of the Gobies 
have a bunch of fibres at one pole which serves to attach them. 
In the Smelt (Osmerus eperlanus) a portion of the outer egg- 
membrane breaks away from the rest and becomes turned back, 
inside out, but remains attached to the egg at one point. By 
means of this membrane the egg is attached to rocks or stones. 
Pelagic eggs are distinguished by their lightness and buoyancy, 
so that they always float near the surface of the water, and by 
their smaller size and remarkable transparency (Fig. 235). A 
conspicuous feature in many of them is the presence of a single 
